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H'i'IiR li f~:: I1lETIC'=.. IF![., ?i~=_. '_?,?• -- ~ <br />• • <br />Tt is our understanding that one purpose of the current study <br />was to develop the necessary data and design a groundwater <br />containment system below the tailings facilities. Since no value <br />for the storage coefficient of the aquifer was established, we <br />cannot model well interference effects or design an effective <br />groundwater containment system. No such design is attempted in the <br />document. <br />The report also addresses the suitability of the existing <br />wells to monitor and intercept contamination, should it reach the <br />groundwater system. The consultant conducted some co~Itaminant <br />transport modeling and determined that the existing wells would <br />intercept any contaminant release. However, we have not been able <br />to reproduce the modeling completed by the consultant and request <br />that additional information and supporting data on the modeling <br />process be provided. <br />In the absence of additional information, it appears to us <br />that groundwater releases in key areas of the tailings facility, <br />the center area of the tailings embankment and the collection pond <br />area, may not be picked up in the existing wells. Lacking any data <br />on aquifer anisotropy, we can assume that groundwater flow is <br />perpendicular to the equipotential lines delineated on Figure 7 of <br />the report. By constructing "reverse flow" lines upgradient and <br />perpendicular to the equipotential lines, we can estimate where <br />water moving by advective flow may have originated. 5y~cri lines <br />drawn upgradient from the three monitor wells are depicted on the <br />attached sketch. These reverse flow lines suggest that monitor <br />wells M-9 and M-13 are most effective in identifying contamination <br />which may originate north of the main body of the tailings <br />embankment. Similarly, well M-12 is most effective in identifying <br />contamination originating from the south end of the tailings. None <br />of the wells appear particularly well suited to identify <br />contamination which may originate from the collection pond, or <br />center of the tailings embankment. <br />it is well understood that dispersion effects will cause a <br />contaminant plume to widen down-gradient. with this in mind, the <br />"reverse flow lines" may be thought of as the axis of plumes which <br />widen to the west. The degree to which the plume will widen as it <br />moves, at least in the modeling, is dependent on the values for <br />dispersivity utilized in the computer code. Although we have not <br />been able to reproduce the transport conditions depicted in the <br />consultants report, they are apparently determining that the plumes <br />will widen adequately down gradient to be recognized in the monitor <br />wells. To accomplish this, a lateral dispersivity of 100 feet, and <br />a transverse dispersivity of 30 feet were utilized in the <br />calculations. These are assumed values, since no measurements were <br />made of dispersivity in the field. In fact, dispersivity is often <br />estimated since it is quite difficult to effectively measure. <br />However, the values used here appear inappropriately high, <br />especially if used for evaluating contaminate plumes between the <br />monitor wells and collection pond. <br />